In many fields of use and, in particular, in the electrical industry, it is necessary to provide a coating on ferrous material. This coating desirably performs the function of separating and purifying the ferrous material and reacting with surface silica in the steel to form an electrical insulating layer. For example, in the transformer art, the cores of the transformers are usually formed of a ferrous material, such as silicon steel, which may be provided with a preferred grain growth orientation to provide optimum electrical and magnetic properties. It has been found necessary to provide a coating on the ferrous material prior to the final high temperature grain growth anneal. This coating will perform three separate functions. The first function of the coating is to provide separation of the various turns or layers of the coiled material to prevent their sticking or welding together during high temperature anneals. A second function is that of aiding in the chemical purification of the ferrous material to develop the desired optimum magnetic characteristics of such material. The third function of the coating is to form on the surface of the ferrous material a refractory-type coating which will provide electrical insulation of one layer of ferrous material from the next during its use as a core in a transformer or in other electrical apparatus, such as motor armatures or the like.
In the present state of the electrical apparatus art, the most widely used coating for the ferrous material which is used as the magnetic core of the electrical apparatus is a coating of magnesium oxide and/or magnesium hydroxide. These coatings are, in general, applied to the ferrous material in the form of a suspension of magnesium oxide and/or magnesium hydroxide in water. The suspension comprises a quantity of magnesium oxide in water and is mixed sufficiently for the desired application: the magnesium oxide being hydrated to an extent dependent on the character of the oxide used, the duration of mixing and the temperature of the suspension. Therefore, the term magnesium oxide coating is with reference to a coating of magnesium hydroxide, which may include magnesium oxide which has not been hydrated.
As set forth in U.S. Pat. No. 2,385,332, during a heat treatment at suitable temperatures, magnesium oxide can be caused to react with silica particles on or near the surfaces of previously oxidized silicon-iron sheet stock to form a glass-like coating, which coating is useful as an interlaminary insulator in the use of silicon-iron in electrical apparatus, e.g. in the cores of transformers.
In the production of silicon steel for the magnetic cores of transformers, the steel is generally annealed to provide optimum grain growth orientation which develops the magnetic properties of the silicon steel. This anneal is usually carried out in a dry hydrogen atmosphere at temperatures ranging from approximately 950.degree. to 1500.degree. C. from about 2 to about 50 hours. This anneal also aids in purifying the steel, aided by the coating placed on the steel. During this anneal, a portion of the magnesium oxide coating reacts with the silica on the surface of the silicon steel to form a glass-like coating of magnesium silicate. This glass-like coating provides electrical insulation during the use of the silicon steel in electrical apparatus, e.g. in the cores of transformers.
U.S. Pat. No. 3,841,925 discloses MgO compositions having a Citric Acid Activity (CAA) of 30 to 85 seconds, a pore volume of 0.20 to 0.1 cc/g and containing small amounts of a chloride contributor, such as MgCl.sub.2, and sodium metasilicate. These compositions are designed to prevent hydration, or formation of magnesium hydroxide from the MgO. The value of this property, insofar as the reference is concerned, is to improve adherence of the dried unannealed coating of MgO, and consequently, the quality of the annealed coating.
British Pat. No. 1,460,943 is also directed to MgO compositions which have a particle size distribution of 92 to 99 percent passing through a 325 mesh screen. This patent is directed to improvement in adhesion of the final annealed film. This patent is silent as to the effect of Citric Acid Activity on the final film. In the instant invention, the Citric Acid Activity was found to be a critical factor in limiting the hydration rate in the aqueous slurry bath.
The instant invention is directed to a MgO composition which eliminates "tight magnesia", or excess MgO which sinters tightly to the annealed coating (glass film) while minimizing the hydration rate in the aqueous coating bath.
A portion of the magnesium oxide coating reacts with the surface silica to form a glass-like magnesium silicate coating. The unreacted portion remains as excess MgO which must be removed prior to further processing. Generally, this removal is accomplished by mechanical scrubbing with nylon bristle brushes or the like. After scrubbing, if there is a residue it is termed "tight magnesia", and is undesirable.
There are, of course, other properties for the annealed coating which must be present, but the composition of this invention is directed to minimizing "tight magnesia" while maintaining all the other desirable characteristics.